1. Field of the Invention
The present invention relates to a light emitting device based on a filled tetrahedral (FT) semiconductor.
2. Description of the Related Art
In recent years, active research is being conducted on band engineering for modulating optical characteristics peculiar to a substance, such as emission and absorption, by modulating the energy band structure that has been considered to be inherent in the substance.
For example, a quantum dot (or a quantum wire or a superlattice) and a strain effect are well known as a typical band engineering technique. The quantum dot (or the quantum wire or the superlattice) brings about a modulated band structure by reducing the size of a substance three-dimensionally (or two-dimensionally or one-dimensionally) and confining electrons therein. The strain effect denotes an effect of modulating a band structure by applying a tensile stress or a compression stress to a substance.
On the other hand, a filled tetrahedral (FT) semiconductor is theoretically proposed as a band engineering method for modulating the band structure of a semiconductor in a quite different principle (see H. W. A. M. Rompa et al., Phys. Rev. Lett., 52, 675 (1984); D. M. Wood et al., Phys. Review B31, 2570 (1985)).
The FT semiconductor is referred to as a solid substance in which a rare gas atom or a diatomic molecule with an electron configuration of a closed shell structure is introduced into the interstitial site of a matrix semiconductor having a tetrahedral structure such as a diamond structure or a zinc blende structure, as shown in FIG. 1.
A difference in the band structure between ordinary crystal silicon and an FT semiconductor will now be described. FIG. 2A is a band diagram of crystalline silicon, and FIG. 2B is a band diagram of silicon doped with He. FIG. 2B shows the result of the first principle band calculation in respect of silicon with an FT structure (hereinafter referred to as an FT-silicon), in which a He atom is imaginarily inserted in the interstitial site of crystalline silicon. As apparent from these diagrams, the band structure the FT-silicon is modulated into a direct transition type well resembling that of GaAs in which the shape of the conduction band is widely varied from that of the crystalline silicon. One of the effects of the FT semiconductor is that an indirect band structure of an indirect semiconductor represented by silicon, that is non-emissive, is greatly modulated into a direct band structure as to exhibit light-emitting characteristics (or transition probability) of a level comparable to those of a direct semiconductor such as GaAs.
However, the rare gas-containing FT semiconductor or molecule-containing FT semiconductor proposed by Rompa et al. is believed to be thermally unstable because the inserted substance can move within the crystal and, thus, not to be suitable for practical use.
Concerning the FT semiconductor, the result of an experiment is reported that, if rare gas atoms are ion-implanted in a silicon wafer, photoluminescence (PL emission) is generated in the energy region in the vicinity of 1 eV, though the mechanism of the PL emission is not clarified (see N. Burger et al., Phys. Rev. Lett., 52, 1645 (1984). However, if the wafer in which the rare gas atoms have been ion-implanted is annealed, the PL emission is caused to disappear, though the reason therefor is again not clear. It is believed that the disappearance of PL emission is derived from the fact that, since the rare gas atom is not chemically bonded with the silicon atom, the rare gas atom is diffused within the silicon crystal and may be finally released from the wafer.
Under the circumstances, it can be easily expected that the rare gas-containing FT semiconductor or molecule-containing FT semiconductor, which can certainly form the FT structure, may be poor in lower thermal stability. In short, there is a problem that the FT semiconductor will not be a practical material system.
As described above, the FT semiconductor as a novel band engineering technique can produce the effect of providing a light-emitting function to an indirect semiconductor. However, there is a problem that the FT semiconductor in which a closed-shell substance such as a rare gas atom or diatomic molecule is inserted into the interstitial site is poor in thermal stability and is not practical because the closed-shell substance is not chemically bonded with the matrix semiconductor.